Conversion of nitrogen dioxide (NO2) to nitric oxide (NO)

ABSTRACT

Various systems, devices, NO 2  absorbents, NO 2  scavengers and NO 2  recuperator for generating nitric oxide are disclosed herein. According to one embodiment, an apparatus for converting nitrogen dioxide to nitric oxide can include a receptacle including an inlet, an outlet, a surface-active material coated with an aqueous solution of ascorbic acid and an absorbent wherein the inlet is configured to receive a gas flow and fluidly communicate the gas flow to the outlet through the surface-active material and the absorbent such that nitrogen dioxide in the gas flow is converted to nitric oxide.

CLAIM OF PRIORITY

This application is a continuation of Ser. No. 13/480,376, filed on May 24, 2012, which is a divisional of U.S. Ser. No. 13/072,466, filed on Mar. 25, 2011, which is a divisional of U.S. Ser. No. 12/361,274, filed Jan. 28, 2009, which claims the benefit of prior U.S. Provisional Application No. 61/024,178, filed on Jan. 28, 2008, each of which is incorporated by reference in its entirety.

TECHNICAL FIELD

This description relates to controlled generation of nitric oxide.

BACKGROUND

Nitric oxide (NO), also known as nitrosyl radical, is a free radical that is an important signaling molecule in pulmonary vessels. Nitric oxide (NO) can moderate pulmonary hypertension caused by elevation of the pulmonary arterial pressure. Inhaling low concentrations of nitric oxide (NO), for example, in the range of 2-100 ppm can rapidly and safely decrease pulmonary hypertension in a mammal by vasodilation of pulmonary vessels.

Some disorders or physiological conditions can be mediated by inhalation of nitric oxide (NO). The use of low concentrations of inhaled nitric oxide (NO) can prevent, reverse, or limit the progression of disorders which can include, but are not limited to, acute pulmonary vasoconstriction, traumatic injury, aspiration or inhalation injury, fat embolism in the lung, acidosis, inflammation of the lung, adult respiratory distress syndrome, acute pulmonary edema, acute mountain sickness, post cardiac surgery acute pulmonary hypertension, persistent pulmonary hypertension of a newborn, perinatal aspiration syndrome, haline membrane disease, acute pulmonary thromboembolism, heparin-protamine reactions, sepsis, asthma and status asthmaticus or hypoxia. Nitric oxide (NO) can also be used to treat chronic pulmonary hypertension, bronchopulmonary dysplasia, chronic pulmonary thromboembolism and idiopathic or primary pulmonary hypertension or chronic hypoxia. Typically, the NO gas is supplied in a bottled gaseous form diluted in nitrogen gas (N₂). Great care has to be taken to prevent the presence of even trace amounts of oxygen (O₂) in the tank of NO gas because the NO, in the presence of O₂, is oxidized to nitrogen dioxide (NO₂). Unlike NO, the part per million levels of NO₂ gas is highly toxic if inhaled and can form nitric and nitrous acid in the lungs.

SUMMARY

In one aspect. an apparatus for converting nitrogen dioxide to nitric oxide includes a receptacle including an inlet, an outlet, a surface-active material coated with an aqueous solution of ascorbic acid and an absorbent, wherein the inlet is configured to receive a gas flow and fluidly communicate the gas flow to the outlet through the surface-active material and the absorbent such that nitrogen dioxide in the gas flow is converted to nitric oxide. The absorbent can be silica gel or activated alumina.

In another aspect, a method of providing a therapeutic amount of nitric oxide to a mammal includes diffusing nitrogen dioxide into a gas flow, exposing the nitrogen dioxide to a surface-active material coated with ascorbic acid and an absorbent to eliminate the by-products of ascorbic acid oxidation and transporting the nitric oxide in a therapeutic amount to a mammal.

In a further aspect, a system of delivering a therapeutic amount of nitric oxide to a mammal includes a gas source of nitric oxide; and a NO₂ scavenger selected from the group consisting of proline and diphenylamine.

In one aspect, a recuperator for converting nitrogen dioxide into nitric oxide includes an exit shell including an outlet, an inside shell wherein the inside shell includes perforated inner and outer tubes with fixed annulus, a surface-active material coated with an aqueous solution of ascorbic acid, an absorbent and a top cap including an inlet wherein the inlet is configured to receive a gas flow and fluidly communicate the gas flow to the outlet through the surface-active material such that nitrogen dioxide in the gas flow is converted to nitric oxide. The recuperator further includes an annular ring around the top cap.

In another aspect, a system for delivering nitric oxide to a patient, includes a gas source of nitrogen dioxide, dinitrogen tetraoxide, or nitric oxide, a first device having an inlet, an outlet, and a porous solid matrix positioned between the inlet and the outlet, wherein the porous solid matrix is coated with an aqueous solution of an antioxidant, and wherein the inlet is configured to receive a gas flow from the source and fluidly communicate the gas flow to the outlet through the porous solid matrix to convert nitrogen dioxide in the gas flow into nitric oxide, and a recuperator coupled to the outlet of the first device, the recuperator converting nitrogen dioxide into nitric oxide prior to delivery to the patient. The recuperator can have a flow resistance of less than 3 cm of water pressure at a flow of 60 L/minute. The recuperator can have a flow resistance of less than 1 cm water at 15 L/min. The recuperator can operate at atmospheric pressure. The recuperator can have an oxygen concentration of in the range of 21 to 100%. The recuperator can have a humidity of dry to 99% (non condensing). The recuperator can be thermally insulated. The recuperator can be coupled to the outlet of a first device through a humidified line. The humidified line can be heated. The humidified line can be heated to about 35° C. The recuperator can be coupled to a NO/NO₂ gas analyzer. The recuperator can further include a particle filter.

In a further aspect, a method of sampling NO and NO₂ gas in a NO delivery system includes obtaining a sample of gas, diluting the sample of gas with air, and measuring the amount of NO and NO₂ gas with a gas analyzer. The sample of gas can be diluted by 50%. The sample of gas can be diluted by 33%. The sample of gas can be diluted with air from the hospital room. The sample of gas can be diluted with air from the wall. The sample of gas can be diluted with oxygen.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWING

FIG. 1 is a block diagram of a cartridge that converts NO₂ to NO.

FIGS. 2A-B are diagrams depicting implementations of a disc filter recuperator.

FIGS. 3A-B are diagrams depicting implementations of a tubular filter recuperator.

FIG. 4 is a diagram depicting an implementation of a recuperator.

FIG. 5 is a diagram depicting another implementation of a recuperator.

FIGS. 6A and B are flow diagrams depicting Nitric Oxide (NO) delivery systems and flow of the gasses.

FIG. 7 is a graph depicting the variation of pressure drop with size of a disc recuperator.

FIG. 8 is a flow diagram depicting nitric oxide delivery for an intensive care unit.

FIGS. 9A-D are diagrams depicting geometrical depictions of dish filters and tube filters.

DETAILED DESCRIPTION

Nitric Oxide (NO) is very well known and well-studied gas. NO is normally present in the atmosphere (as a pollutant from automobiles and power plants) at concentrations between 0.010 and 0.500 parts per million (ppm), and NO concentrations may reach 1.5 ppm in heavy traffic. NO is also present in tobacco smoke at levels as high as 500 ppm to 2000 Ppm.

For medical applications, NO gas, like oxygen has been studied and used to treat patients for many years. In biological systems, NO is a molecule that is naturally produced in the human body. NO is one of the few gaseous signaling molecules known. NO is a key vertebrate biological messenger, playing a role in a variety of biological processes. NO is highly reactive (having a lifetime of a few seconds), yet diffuses freely across membranes. These attributes make NO ideal for a transient signal molecule between adjacent cells and within cells. Several pharmaceutical products, such as Nitroglycerin, amyl nitrite and Sildenafil (Viagra) serve as vasodilators because they either release or cause NO to be released in the body. In 1987, the biologic similarities of NO to endothelium-derived relaxing factor were demonstrated. Subsequently, NO and endothelium-derived relaxing factor were considered the same entity. During the late 1980s and early 1990s, inhaled NO emerged as a potential therapy for the acute respiratory distress syndrome (ARDS), sickle cell anemia, COPD and other conditions. Since then NO has been shown to reduce persistent pulmonary hypertension and also to reduce pulmonary hypertension without any undesired drop in systemic blood pressure, which is valuable when treating heart and lung transplant patients and other patients having undergone interventional cardiovascular procedures. The gas was readily available for many years from several suppliers as were several competing CE marked delivery systems. During the 1990s the medical use of the gas was patented and the cost has increased substantially. Even with this restriction, NO is currently routinely and safely used under institutional or countywide protocols for many uses outside of the approved indications for neonates.

When delivering NO for therapeutic use to a mammal, it can be important to avoid delivery of nitrogen dioxide NO₂ to the mammal. NO₂ can be formed by the oxidation of NO with oxygen (O₂). The rate of formation of NO₂ is proportional to the O₂ concentration multiplied by the square of the NO concentration—that is, (O₂)*(NO)*(NO)=NO₂.

In one aspect, a NO delivery system that converts NO₂ to NO is provided. The system employs a surface-active material coated with an aqueous solution of antioxidant as a simple and effective mechanism for making the conversion. One example of a surface-active material is silica gel. Another example of a surface-active material that could be used is cotton. The surface-active material may be or may include a substrate capable of retaining water. Another type of surface-active material that has a large surface area that is capable of absorbing moisture also may be used. More particularly, NO₂ can be converted to NO by passing the dilute gaseous NO₂ over a surface-active material coated with an aqueous solution of antioxidant. When the aqueous antioxidant is ascorbic acid (that is, vitamin C), the reaction is quantitative at ambient temperatures.

The oxidation of ascorbic acid with oxygen under moist conditions can be complex, with over 50 different compounds having been reported. (See J. C. Deutsch, “Spontaneous hydrolysis and dehydration of dehydroa,” Analytical Biochemistry, Vol. 260, no. 2, pages 223-229 (Jul. 1, 1998); Dong Bum Shin and Milton S. Feather, “3-deoxy-L-glycero-pentos-2-ulose (3-deoxy-L-xylosone) and L-threo-pentos-2-ulose (L-xylosone) as intermediates in the degradation of L-ascorbic acid,” Carbohydrate Research, Vol. 280, pages 246-250 (Dec. 15, 1990); Eiji Kimoto et al., “Analysis of the transformation products of dehydro-L-ascorbic acid by ion-pairing high-performance liquid chromatography,” Analytical Biochemistry, Vol. 214, pages 38-44 (1993), Academic Press; Steven R. Tannenbaum et al., “Inhibition of nitrosamine formation by ascorbic acid,” The American Journal of Clinical Nutrition, Vol. 53 (1 Suppl.) pages 247S-250S (January 1990), all of which are incorporated by reference in their entireties). The reaction generally leads to dehydroxy ascorbic acid, which can then be further degraded into multiple species.

FIG. 1 illustrates an example of a cartridge 100 for generating NO by converting NO₂ to NO. The cartridge 100, which may be referred to as a NO generation cartridge, a cartridge, or a cylinder, includes an inlet 105 and an outlet 110. Screen and glass wool 115 are located at both the inlet 105 and the outlet 110, and the remainder of the cartridge 100 is filled with a surface-active material 120 that is soaked with a saturated solution of antioxidant in water to coat the surface-active material. The surface-active material can be silica gel. The screen and glass wool 115 also is soaked with the saturated solution of antioxidant in water before being inserted into the cartridge 100. In the example of FIG. 1, the antioxidant can include ascorbic acid. In other embodiments, the antioxidant can include alpha tocopherol or gamma tocopherol.

The moist silica gel of the cartridge can adsorb and bind up the vast majority of the products of the side reactions.

In the presence of moisture, oxygen and NO, NO₂ forms N₂O₃, N₂O₄ and the nitrite ion. In one embodiment, these reactants can combine with an NO₂ scavenger which can include the common amino acid, proline, to form N-nitroso proline. N-nitrosproline is non carcinogenic. This reaction has been used in vivo by Tannenbaum The American Journal of Clinical Nutrition, Vol. 53 (1 Suppl.) pages 247S-250S (January 1990), and Ohshima and Bartsch (Cancer Res. Vol. 41, p. 3658-3662 (1981), to measure the nitrosation capacity of the body, and to show that the addition of Vitamin C can reduce this capacity. Such a reaction can be used to trap out NO₂ in the gas phase from an air stream containing moist NO₂ in the presence of oxygen and NO. The proline can be in the form of a crystalline powder. Proline can be placed in a tube and gas can be allowed to flow over it. The NO₂ that is present can bind irreversibly with the proline to form N-nitroso proline. The application of this reaction is to use this reaction as a scavenger to remove the last minute traces of NO₂ from an air stream containing NO and oxygen and air. In one embodiment, such a NO₂ scavenger can be used in a NO delivery system to allow any NO₂ that is present to bind irreversibly with the proline to form N-nitroso proline.

The proline can be in the form of a powder, or as a solution that has been deposited onto a substrate such as silica gel, activated alumina and charcoal. Other appropriate substrates can be used as long as proline is available to react with NO₂ gas. In one embodiment, an aqueous solution of proline in water can be used. In another embodiment, diphenylamine or any secondary or tertiary amine can be used to react with NO₂ gas. Examples of secondary amines can include dimethylamine, methylethanolamine or 2-(methylamino)ethanol, cyclic amines such as aziridine, azetidine, pyrrolidine and piperidine. Examples of tertiary amine can include trimethylamine, dimethylethanolamine (DMEA), 2-(dimethylamino)ethanol or bis-tris.

Preferably, any material can be used where the Nitroso product will not be carcinogenic or toxic. In other embodiments, any compounds that bind with the NO₂ to form organic compounds can be used. Products include but are not limited to: nitro, nitroso, or azo as long as the NO₂ is chemically bound up so as to remove it from the system.

In one embodiment, an NO₂ scavenger can be included in a NO delivery system. The purpose of the NO₂ scavenger is to remove any NO₂ gas that may have been formed in the ventilator and during storage in a gas bag or other temporary gas storage device. In another embodiment, the NO₂ scavenger can remove NO₂ that is formed in the gas plumbing lines from the exit of the NO generation cartridge. The NO₂ scavenger can serve as a safety device to reduce the NO₂ levels to below 0.1 ppm, at any flow and at any NO concentration, prior to delivery to a patient.

In another aspect, a recuperator is included in the NO delivery system. The purpose of the recuperator is to convert any NO₂ gas that may have been formed in the ventilator and during storage in a gas bag or other temporary gas storage device to NO. In one embodiment, the recuperator is a device that is immediately adjacent to the patient. It serves the same purpose as the main NO generation cartridge, namely to convert NO₂ to NO. In another embodiment, the recuperator coverts NO₂ that is formed in the gas plumbing lines from the exit of the cylinder NO generation cartridge to NO. The recuperator can be a cartridge that is needed to recover any NO₂ that was formed in the ventilator and in the gas lines from the reaction of NO and Oxygen. The recuperator can serve as a safety device to reduce the NO₂ levels to below 0.1 ppm, at any flow and at any NO concentration, prior to delivery to a patient.

In one embodiment, the flow resistance for the recuperator can be as low as possible, for example, less than 3 cm of water pressure at a flow of 60 L/minute, and/or <1 cm water at 15 L/min. The recuperator can operate at atmospheric pressure. The oxygen concentration at the recuperator can be in the range of 21% to 100%. The humidity at the recuperator can be 0% to 99%. The recuperator can be thermally insulated to prevent water condensation. The inlet to the recuperator can be from the humidified (and heated) line that is delivering gas to the patient. This line can be typically heated to about 35° C. to prevent water condensation in the lines.

The exit side of the recuperator can be a sample probe that goes to the NO/NO₂ gas analyzer. The sample line can be diluted with an equal volume of air so as to reduce the relative humidity and to minimize the rate of formation of NO₂ from Oxygen in the sample line to the analyzer. The weight of the recuperator can be kept as low as possible, so that it is not unwieldy, under 2 pounds but preferably under 1 pound or under 0.5 pounds. The exit from the recuperator can go directly to the patient by means of a short (approximately 6 inch) length of roughly 1 inch diameter ventilator tubing. The recuperator can be disposable and can be for a single use only. The recuperator can incorporate a particle filter to prevent any possibility that trace amounts of dust from the recuperator being delivered to a patient, for example, fine silica gel or ascorbic acid for the recuperator, or fine proline for the scavenger. The same filter material will also prevent bacteria and other particles from being delivered to the patient.

In one embodiment, the recuperator is provided as a disc as exemplified in FIGS. 2A and B. In a further embodiment, the recuperator can include a common filter design that is widely used in industry which is a tubular design with co-axial tubes. This type of design is especially common in water filters and for use in compressed in air lines. In another embodiment, the recuperator is provided as a tubular filter as exemplified in FIGS. 3A and B. As depicted in FIGS. 3A and B, the tubular filter can be constructed from three concentric tubes, with the filter medium being held tightly in place in a perforated section of the interior. The tubular filter can hold the silica gel or ascorbic acid dust in place in the annulus between the two perforated tubes. A filter medium can be placed on both contact sides of the silica gel or ascorbic acid dust. The powder or dust can be compressed during filing without the compression material coming into contact with the flowing air stream. The aspect ratio will be easier to handle adjacent to the patient, where a small diameter shape can be used. The tubular filter can include an exit shell as depicted in FIG. 3B, an inside shell that includes perforated inner and outer tubes with fixed annulus and an annulus filled with silica gel or ascorbic acid dust. The inner and outer annulus can be lined with filter material. In one embodiment, the final assembly of the tubular filter is depicted in FIG. 3B. The final assembly can include a top cap and an annular ring at the top can keep pressure on the silica gel or ascorbic acid dust.

In one embodiment, a method of NO and NO₂ gas sampling is provided. For example, for 100 μm of NO and 100% oxygen, 1.70 ppm of NO₂ can be formed in the gas sampling lines from the reaction of NO with oxygen. The problem is how to sample for NO₂ in a gas stream that has the reaction of NO and oxygen going on at the same time. This is made worse at high NO concentrations. For example at 200 ppm NO the rate of formation of NO₂ in the sample line is 4 times the rate as compared to 100 ppm. Also at 100% oxygen the rate is 5 times the rate in air. To get an accurate reading of what was in the line at the patient there is a need to either sample quickly, or slow the reaction down somehow. By diluting by 50%, the rate is decreased 4 fold due to the drop in NO concentration and approximately 2.5 fold by the drop in Oxygen concentration.

In one embodiment, the sample tube from the patient to the detector can be diluted up to 66%, or up to 50% or up to 33%. The sample can be diluted at the sample point. The sample can be diluted with air. For example, dilution of one part sample and one part with air can reduce the water concentration in the sample. In another embodiment, the sample can also be diluted as follows: one part sample and two parts with air from the room (e.g. hospital room). Alternatively, the sample can also be diluted using bone dry air from the wall.

In a further embodiment, the sample tube can be spliced in two or three, adjacent to the sampling point on the recuperator. This can be done for 50 or 66% dilution.

EXAMPLES Gas Cylinder Cartridge Design

In another aspect, the gas cylinder and appropriate amount of NO₂ for clinical use is provided. The FDA standard room size: 3.1×6.2×4.65 m=89.3 m^(3=89,300) L. The OSHA NO₂ level is 5 ppm. All the three gas cylinders described herein are approximately equivalent in the amount of gas that they can deliver.

Size AS of the cylinder is pressurized to 2000 psi. The sudden release of the entire contents of 3600 L of 124 ppm would lead to a NO₂ level of 5.0 ppm in the room, if there was no air exchange. Thus, in order to meet the current FDA requirement for safety, the highest concentration in a gas cylinder of this size and type should be 100 ppm of NO₂ (with a built in safety factor). The cylinder that is used in the lab will deliver 3,600 liters of gas (without dilution) containing 100 ppm of NO₂. At 5 L/min, this gas cylinder will last for 720 minutes=12 hours. At 48 pounds, without the regulator and top, this cylinder is far too heavy to be picked up by a nurse or therapist, and has to be moved on a wheeled cart.

Size AQ/BL188 2000 psi cylinder is currently in use in hospitals. The sudden release of the entire contents of 1918 L of 233 ppm would lead to a NO₂ level in the hypothetical room of 5.0 ppm. Thus, in order to meet the current FDA requirement for safety, the highest concentration in a gas cylinder of this size and type should be 200 ppm of NO₂ (with a built in safety factor). For a cylinder of this size and 200 ppm of NO₂, the ideal oxygen level would be 70-74%. This pressure cylinder will deliver 3836 liters of gas (after dilution) containing 100 ppm of NO₂. At 5 L/min, this gas cylinder will last 767 minutes=12.8 hours. This cylinder weighs 30 pounds and is still too heavy to be picked up by a nurse. It is a bit more maneuverable but still needs a wheeled cart for transport.

Luxfer's ME36 3000 psi cylinder holds 992 Liters at a pressure of 3000 psi instead of 2000 psi. The sudden release of the entire contents of 992 L of 450 ppm would lead to a NO₂ level of 5.0 ppm in the hypothetical room. Thus, in order to meet the current FDA requirement for safety, the highest concentration in a gas cylinder of this size and type should be 400 ppm of NO₂ (with a built in safety factor). This cylinder however, weighs only 8.3 pounds compared to INO's 30 pounds. The contents of this small, high pressure cylinder would last as long as the AQ and the AS. This translates into the cylinders being used up at twice or four times the rate of INO but at less than ⅓^(rd) of the weight this is a reasonable trade off. The key advantage is that the cylinder is small and light enough to be stocked in a pharmacy and picked up by a nurse with one hand. The ideal oxygen level in a cylinder of this size would be about 60%. The Luxfer high pressure miniature gas cylinder will deliver 3968 liters of gas (with dilution) containing 100 ppm of NO₂. At 5 L/min, this gas cylinder will last 793 minutes=13.2 hours. At only 8.3 pounds, this cylinder can be picked up by a nurse with one hand. It is small enough to be stored in a hospital pharmacy. The 3000 PSI Luxfer Cylinder offers the best performance and is the preferred package. Table 1 shows the specifications of the cylinders described herein.

The physical layout of the recuperator is able to accommodate the 4.5 inch diameter cylinder, and an output tube that contains activated charcoal powder. As an example, a tube design is exemplified in FIG. 4.

The revised design depicted in FIG. 5 has the two main tubes as close together as possible, with the small vertical tubes tucked in close. The entire package has to fit inside a 4.5 inch or less gas cylinder top. This is shown schematically in FIG. 5. Each main tube that holds the ascorbic acid/silica gel dust has an inside diameter of about 1 inch. The inlet and outlet tubes need to be on the same side. The short tube can contain a small amount of activated charcoal to remove traces of acetaldehyde. Several embodiments of the entire package for use in an Intensive Care Unit are depicted in FIGS. 6A and B.

The gas bottle contains a mixture of 60% oxygen with the balance being N₂. The gas also contains about 400 ppm of NO₂. This gas leaves the gas cylinder through a built in regulator where the pressure is reduced to the 20 to 100 psi level. The gas is attached to a separate blending box by means of a unique quick disconnect.

The gas containing 400 ppm NO₂ is then blended with an air/oxygen mixture to reduce the NO₂ concentration to the therapeutic concentration. In current use, this is 0.1 to 80 ppm. In one aspect of the system, this could be extended upwards to >200 ppm. The blender dial is calibrated in ppm equivalents of NO. The gas leaving the blender flows onto the NO generation cartridge by means of a quick disconnect attachment. The air oxygen blender is a conventional design and is available commercially. The air and oxygen are typically supplied from the hospital wall supply as a utility. Alternatively, the gas can flow through the ventilator first before the cartridge to reduce the gas pressure from 50 to 20 psa.

The cartridge converts the NO₂ to NO. As the gas leaves the cartridge, the gas now has NO at the proper therapeutic concentration in an air oxygen blend of the appropriate oxygen concentration. The gas leaving the cartridge is connected back to the blending box by means of a quick disconnect fitting, where the oxygen concentration can be sampled and displayed. In one embodiment, an oxygen sensor can be used to precisely set the appropriate oxygen concentration.

The reason for the three connections to the blender box is to allow quick replacement of the gas cylinder. A second cylinder will be plumbed to an identical set of three quick disconnect fittings on the blender box. When a cylinder needs to be changed, a single three stack valve is used to switch from one gas cylinder to another, allowing for the empty cylinder to be replaced. This is not shown on the FIG. 6A.

The gas mixture from the blender box becomes replaces the oxygen feed on a conventional medical ventilator. The ventilator is then used in its conventional mode and can perform whatever ventilatory cycles that the therapist desires for a particular patient. The device is intended to provide the physician a mechanism for delivering a low concentration (dose) of pure NO gas in a mixture of oxygen and air. The gas passes through a mechanical or manual ventilator and travels through respiratory tubing and a mask or tube to the patient's lungs. The gas flow can be regulated by a mechanical ventilator or manual ventilation or by delivery directly to the patient from the pressurized gas tank (for spontaneously breathing patients). The device allows the physician to independently adjust the NO concentration and the oxygen concentration of the delivered gas.

Indication for Use

The device is indicated to provide pure NO gas at different concentrations in an oxygen/air mixture.

General Product Description

In one embodiment, The Nitric Oxide (NO) Generator and Delivery System include five components that work together to create, deliver and monitor pure Nitric Oxide (NO) gas in an oxygen/air mixture. The gas travels through standard anesthesia and respiratory breathing devices for inhalation by the patient. The anesthesia part is only needed to provide variable flow rates and/or to assist patients who are not breathing on their own. In its simplest form the gas bottle stands alone and the gas is converted to NO as it leaves the gas bottle. With this approach the gas is then fed into a mask or a cannula. The gas flow is provided by mechanical or manual ventilation or by the pressurization of the tank (for spontaneously breathing patients). The concentration of NO and Oxygen are determined and adjusted by the physician based on each patient's condition and needs.

Product Components

Gas Tanks

The first component is a pressurized aluminum gas tank with a small quantity of Nitrogen Dioxide (NO₂) gas in an Oxygen/Air mixture. This mixture cannot be inhaled without processing by the other components as it would be toxic. The tanks will come with a standard regulator to limit the pressure of the gas to the mixer. Tanks will have concentrated level of NO₂ gas and in a fixed Oxygen/Air ratio. These concentrations will be adjusted using the mixing system below. Tanks used or transport will be at set concentrations of NO₂ gas and Oxygen/Air and will not require mixing.

Mixing System

The mixing system includes two standard gas blenders that are connected and an oxygen sensor. The first mixing chamber takes medical oxygen and air, which can be provided from pressurized tanks or the hospital's gas system. A knob allows selection of the desired FiO₂ (fraction of inspired oxygen) of the gas to be delivered to the patient which adjusts the oxygen/air mixture in the mixing chamber as measured by the oxygen sensor. The output of this mixture is fed into a second mixing chamber where it is mixed with the NO₂ gas from the gas tank described above. The knob to this mixing chamber allows the physician to select the concentration of NO gas to be delivered to the patient. The output of this mixing chamber is the passed though the gas converter and purification cartridge described below. This allows variable NO and oxygen concentration levels which are independent of each other.

Gas Converter and Purification Cartridge

The gas mixture from the last mixer will flow through the Gas Converter and Purification cartridge. This cartridge will convert all NO₂ gas into NO gas and remove any impurities in the entire gas mixture. The concentration of NO gas will be the same as the concentration of the NO₂ gas as the conversion is essentially 100%. The concentration of the oxygen (oxygen/air ratio) will not be changed. The output of the cartridge will be delivered to the mechanical or manual ventilation system and appropriate pressures. Tanks used for transport will also be fitted with a flow meter to regulate the flow of the gas to the patient.

Recuperator Cartridge

The Recuperator cartridge will be placed at the patient end of the inspiratory limb of the patient's breathing tubing. This cartridge will contain the same technology as the Gas Converter The purpose of this cartridge is twofold. First, it will reconvert any NO₂ gas back into NO that may have formed through the reaction of the NO gas with oxygen. Second it provides bacterial and viral filtration of the delivered gas.

Disc Recuperator

The obvious format for the recuperator is to make it much like the gas cylinder device, but with the diameter of the order of 3 to 4 inches, and the cartridge depth reduced from 5.5 inches to less than 0.3 inches. A cartridge like this has a pancake shape and would look similar to the particle filters that are used with some respiratory equipment.

The equations below show how the pressure drop across the cartridge will vary as a function of radius and the depth. For a constant volume cartridge, the pressure drop varies with the fourth power of the radius. V ₁ =πr ₁ ² l ₁ l ₁ =V ₁ /πr ₁ ²

$p \propto {{l_{i}/\pi}\; r_{1}^{2}} \propto \frac{V_{1}}{\pi\; r_{1}^{2} \times \pi\; r_{1}^{2}} \propto \frac{V}{\pi^{2}r_{1}^{4}}$ $\frac{p_{2}}{p_{1}} = {\frac{\pi \times \pi\; V_{2}}{V_{1} \times \pi \times \pi}\frac{r_{1}^{2} \times r_{1}^{2}}{r_{2}^{2} \times r_{3}^{2}}}$

For constant volume of material

$\frac{p_{2}}{p_{1}} = \left( \frac{r_{1}}{r_{2}} \right)^{4}$

If volume material is half, then

$\frac{p_{2}}{p_{1}} = {0.5\left( \frac{r_{1}}{r_{2}} \right)^{4}}$

The gas bottle cartridge, which has a radius of 0.4 inches and a depth of 5.5 inches, has a pressure drop that was measured experimentally of 2.7 psi=187 cm H₂O water. The calculations of pressure drop for various diameters are shown in Table 2. A pressure drop of 0.2-0.3 cm water at 5 L/min is needed to attain the design goal of 3.0 cm H₂O at 60 L/min. In order to achieve this low a pressure drop, the diameter of the flat disc would need to be 4.0 to 4.5 inches. If it were to have the same amount of material as the current cylinder cartridge, the depth would need to be 0.56 cm at 4.00 inches and 0.44 inches at 4.5 inches.

A flat disc of 3 inches diameter and a 1.0 cm thickness, has been tested in the laboratory and has been shown to perform as well as the cylinder cartridge. The variation of pressure drop with size, where all the filters have the same volume of material, is shown in FIG. 7. Various concepts have been evaluated on how to build such a device. The difficulty is how to encapsulate the silica gel or ascorbic acid dust between two very thin filter cloths, and have not only uniform thickness everywhere, but also no settling of the silica gel or ascorbic acid dust. Settling would be catastrophic and could lead to channeling and failure. An example of one such design is shown in FIG. 2B.

A comparison of the pressure drop across the disc and tubular filters are shown mathematically below and in FIGS. 9A and B.

Dish Filter Tube Filter A = πr² A = 2πr · l V = πr²t V = 2πr · l · t $t = \frac{V}{2\;\pi\;{rl}}$

For Cylinder:

${p \propto \frac{t}{A}} = {\frac{t}{2\;\pi\;{rl}} = {\frac{V}{\left( {2\;\pi\;{rl}} \right)^{2}} = \frac{V}{4\;\pi^{2}r^{2}l^{2}}}}$

For Dish:

$p \propto \frac{V}{\pi^{2}r^{2}r^{2}}$

Essentially, the analysis shows that the pressure drop of the tubular filter, like the disc filter, is proportional to the surface area (the inner circumference of the tubular filter) and the thickness of the bed. The detailed calculation of size and pressure drop are shown next and in FIGS. 9C and D.

Tube Calculation Continued

Volume of Outer Tube: V ₂ =πr ₂₁ ² l

Volume of Inner Tube: V ₁ =πr ₁ ² l

V=Volume of silica gel or ascorbic acid Dust= V ₂ −V ₁ =πr ₂ ² l−πr ₁ ² l

$\begin{matrix} {r_{2} = \sqrt{\frac{Volume}{\pi\; l} + \frac{\pi\; r_{1}^{2}l}{\pi\; l}}} \\ {= \sqrt{\frac{Volume}{\pi\; l} + r_{1}^{2}}} \end{matrix}$

For Volume=2.76 cubic inches (5.5 inches−0.8 inches cylinder)

r_(1=0.5) inch

l=3.0 inches

$r_{2} = {\sqrt{\frac{2.76}{\pi \times 3} + (0.5)^{2}} = {\sqrt{0.29 + 0.25} = {\sqrt{0.54} = 0.73}}}$

gap=0.23 inches≡0.58 cm Area=2πrl=2π(0.5)(3)=94 sq. inches

Equivalent area to disc 3.5⁴ perimeter

Pressure drop=0.5 cm H₂O

These equations were then used to evaluate a variety of shaped tubular filters and compared to the disc filter. For example, a 4 inch long tubular filter with an internal radius of 0.5 inch (1 inch id) and an outer annulus diameter of diameter of 1.25 inches would have an outer shell diameter of about 1.75 inches. A tubular filter with this aspect ration would have a pressure drop at 5 l/min of only 0.19 cm H₂O, which is equivalent to a 4.5 inch diameter disc. The gap between the tubes, called the annulus, would have a spacing of 0.4 cm. See Table 3 From Table 3, assume that the performance of a disc that is 5 inches in diameter is wanted, which would have an effective surface area of 19.63 sq inches and a pressure drop of 0.12 cm of water. In a tubular version, the same surface area and pressure drop can be achieved with a inside diameter of 0.57 inches and an od of 0.70 inches, provided that the tube was 5.50 inches in length.

Gas Monitoring

The system may require gas sensors to monitor and display the concentration of NO and NO₂ that is delivered to the patient. These can be commercially available monitors and should be equipped with alarm capability. A figurative representation of the system is shown on FIG. 8.

For 100 ppm of NO and 100% oxygen, it is shown by both experiment and calculation that 1.70 ppm of NO₂ is formed by the time the sample passes through about 2 meters of tubing, thru a large volume water drop out filter and into the PrinterNox, which is a commercial electrochemical gas analyzer for measuring NO and NO₂ for inhalation applications. The sample can also be >100 saturated with water and the water drop out filter is essential. This is a typical problem that is encountered in stack monitoring from incinerator and power plant smoke stacks. There are several possible solutions:

First, heat the sample lines to keep the water in the vapor phase. If the instrument also runs hot, then the water filter can be eliminated and the sampling time reduced, thereby reducing the NO₂ formation. It is not a good approach for NO₂ sampling since the rate of formation is linear with time and square power with NO₂.

Second, sample at the source. This does work, but the condensing water issue remains.

Third, dilute the sample. This does three critical things:

-   -   1. It dilutes the sample which reduces the formation of NO₂ from         NO and O₂. This makes sampling down lines possible otherwise         most of the NO that is measured at the analyzer will be formed         in the sampling lines.     -   2. It decreases the humidity, which prevents condensation of         water in the lines and thereby eliminates the need for a water         drop out filter.     -   3. It reduces the NO level by 50% which brings the machine into         the working range of the PrinterNOx detection cells

Consider a sample tube with dilution of one (50%).

-   -   Initial conditions: 100 ppm NO, 100% oxygen and condensing water         (>100%)     -   With 50% dilution: 50 ppm NO, 60.5% oxygen, and greatly reduced         humidity.     -   At detector: Rate of formation of NO₂ reduced by 4*100/60.5=6.6         NO₂ reduced from 1.70 to 0.26 ppm

With removal of the large volume water condensation filter volume, the level will come down even further.

NO readings are reduced at all concentrations by 50%. This means 20 ppm reads as 10, and 2 ppm reads as 1 ppm. This is corrected for by calibration, but the precision will be reduced by 50%.

NO₂ formation in the lines is effectively reduced to zero at normal concentrations. For example, even at 80 ppm NO, the NO₂ formed in the lines would be 1.09 ppm without dilution and <0.16 ppm with dilution.

Consider a sample tube with dilution of two (33%).

-   -   Initial conditions: 100 ppm NO, 100% oxygen and condensing water         (>100%)     -   With 33% dilution: 33 ppm NO, 47% oxygen, and greatly reduced         humidity.     -   At detector: Rate of formation of NO₂ reduced by 9*100/47=20 NO₂         reduced from 1.70 to 0.09 ppm

NO readings are reduced at all concentrations by ⅓rd. This means 20 ppm reads as 6.67, and 2 ppm reads as 0.67 ppm. This is corrected for by calibration, but the precision will be reduced by ⅓^(rd).

NO₂ formation in the lines is effectively reduced to zero at normal concentrations. For example, even at 80 ppm NO, the NO₂ formed in the lines would be 1.09 ppm without dilution and <0.05 ppm with dilution.

Humidity

At 37° C., body temperature, the amount of water at 100% relative humidity is 44 g/M³. The air in a hospital is typically at 50% relative humidity and a temperature of 22° C. Air at this temperature contains 10 g/M³.

Diluting one part sample and one part with air from the hospital room would reduce the water concentration in the tube down to 27 g/M³. This amount of water vapor would begin to condense out of the air at a temperature of 28° C. (82° F.).

Diluting one part sample and two parts with air from the hospital room would reduce the water concentration in the tube down to 21.3 g/M³. This amount of water vapor would begin to condense out of the air at a temperature of 23.5° C. (74° F.).

An alternative approach would be to use bone dry air from the wall. This will be available in the same box and could be piped to the sample location by means of a parallel tube.

The detectors are required to run at constant temperature so as to ensure stability. Thus the inside of the detection module would be warm. A 50:50 dilution would work as long as the sample lines were insulated by a thick wall or by having one tube run inside another.

Application (for 50% or 66% Dilution)

The sample tube could be spliced in two (or three), adjacent to the sampling point on the recuperator. Alternatively, the second sampling orifice could be molded into a special adaptor. It would be best not to have a flapping air sample port, since it would raise too many questions from users. This would allow for taking half the sample from the patient inspiratory line and half the sample from the room. Calibration would also have to be at this point. Technically, this would be a perfectly valid way to operate and would meet all regulatory approval guidelines. Naturally, the approach be described in regulatory submissions which would also reference the EPA standard procedures.

Dilution at the sample point is a perfectly viable approach. There would also probably be no need to have the large dead volume water condensation trap on the detector. This would reduce the NO₂ formation level even further, by as much as a factor of 5. The life of detector cells would increase from days to about 12 months, even during in house testing. Dilution is the preferred method of sampling the reactive gas stream.

Other Applications

The gas bottle alone can be used for all applications of NO. It is available to deliver the gas without any electronics whatsoever. The advantages of the system are simplicity, no mixing, no electronics and no software. Just connect the regulator and open the valve.

The gas bottle system can also be used with a dilutor. In this case the gas would be shipped as say 1000 ppm of NO₂ in oxygen. In a first stage, the user's equipment would then dilute this concentration down to say 20 ppm NO₂. The second stage would be to insert the cartridge and convert to NO. this format would be similar to what is currently marketed, but would not require the user to worry about any NO₂ that was formed in the gas lines since it would be removed by the recuperator. Similarly, the recuperator cartridge could be used with existing system to convert all of the residual NO₂ gas being inhaled into the therapeutic form, namely NO. The recuperator also ensures that no NO gas is lost from the system and that the patient is receiving the full prescribed dose.

The fact that the system can deliver high doses of NO, of the order of 100 to 200 ppm or even higher, without the presence of the toxic form, NO₂, may be important. Much of the earlier work was done at doses in the 20 ppm range, but the researchers were always plagued by the presence of toxic NO₂. This limited the does that they could go to. With the system all of the NO₂ toxicity problems in the inhaled gas are eliminated. This fact alone will greatly increase the utility of NO gas for treatment of a multitude of diseases, and especially ARDS (Acute respiratory distress syndrome).

Other implementations are within the scope of the following claims.

TABLE 1 Internal Max Height Diameter Volume Weight Volume Conc Name Inches Inches L Pounds Pressure L PPM LAB AS 48 8 29.45 48 2000 3600 100(124) INCURRENT AQ/BL/88 33 7.25 15.57 30 2000 1918 200(233) USE LUXFER ME36 25.3 4.4 4.5 8.3 3000 992 400(450)

TABLE 2 DISC RECUPERATOR DIAM RADIUS PRESS D r DROP mm THICKNESS INCHES INCHES r² AREA r⁴ cm H₂O H2O INCHES CM Base case 0.80 0.40 0.160 0.503 0.0256 187.00 1870.00 5.5 13.97 1.00 0.50 0.250 0.785 0.0625 76.60 765.95 3.52 8.94 1.50 0.75 0.563 1.767 0.3164 15.13 151.30 1.56 3.97 2.00 1.00 1.000 3.141 1.0000 4.79 47.87 0.88 2.24 2.50 1.25 1.563 4.908 2.4414 1.96 19.61 0.56 1.43 3.00 1.50 2.250 7.068 5.0625 0.95 9.46 0.39 0.99 3.50 1.75 3.063 9.621 9.3789 0.51 5.10 0.29 0.73 4.00 2.00 4.000 12.566 16.0000 0.30 2.99 0.22 0.56 4.50 2.25 5.063 15.903 25.6289 0.19 1.87 0.17 0.44 5.00 2.50 6.250 19.634 39.0625 0.12 1.23 0.14 0.36 const volume

TABLE 3 DISC CYL CYL CYL DISC DISC PRESS CYL INSIDE OUTSIDE CYL CYL PRESS diam AREA DROP AREA rad rad LENGTH GAP DROP INCH sq inch cm-H20 sq in inch inch inch cm cm H2O 2.00 3.14 4.79 2.50 4.91 1.96 3.00 7.07 0.95 7.23 0.50 0.79 2.30 0.75 0.95 3.50 9.62 0.51 9.42 0.50 0.74 3.00 0.60 0.51 4.00 12.57 0.30 12.57 0.50 0.69 4.00 0.47 0.30 4.50 15.90 0.19 15.58 0.62 0.78 4.00 0.40 0.19 5.00 19.63 0.12 19.70 0.57 0.70 5.50 0.32 0.12 

What is claimed is:
 1. A system of delivering a therapeutic amount of nitric oxide to a mammal comprising: a ventilator, an NO₂ scavenger selected from the group consisting of proline, diphenylamine, a secondary amine and a tertiary amine, and a receptacle including an inlet, an outlet, and a surface-active material; wherein the surface-active material includes an absorbent and a reducing agent; and wherein the inlet is configured to receive a gas flow and fluidly communicate the gas flow to the outlet through the surface-active material, such that nitrogen dioxide in the gas flow is converted to nitric oxide.
 2. The system of claim 1, wherein the absorbent is silica gel.
 3. The system of claim 1, wherein the absorbent is activated alumina.
 4. The system of claim 1, further comprising a gas source of nitrogen dioxide, dinitrogen tetraoxide, or nitric oxide.
 5. The system of claim 1, further comprising a gas source of nitric oxide.
 6. The system of claim 1, wherein the receptacle includes the NO₂ scavenger.
 7. The system of claim 1, further comprising a humidified line.
 8. The system of claim 7, wherein the humidified line is heated.
 9. The system of claim 7, wherein the humidified line is heated to about 35° C. 